Multilayer laminated film form composite structures are well known in the plastics art. One wide spread use of these composite materials is to form them into bags or pouches adapted to hold granular material, such as for example rice or the like.
In this and other uses, it is often desirable to provide printing or other indicia for the ultimate package, that is the bag or pouch, on the film or films to be laminated together to form the composite film structure. It is obvious that the surfaces of the films, and anything disposed thereon, such as printing ink, or other indicia, are liable to be marred, either by abrasion with the contents of the package or by abrasion by contact with things which are external to the package, at least to the extent that such surfaces contact the bag contents.
Therefore, it is common practice to provide the printing on the surface of the film(s) which will be disposed between the laminated layers of composite film which makes up the package. This acts to protect the printing by interposing a film of plastic between the printing ink and the bag contents as well as between the printing ink and things external to the bag which might rub against the bag and thus damage the printed images.
In this use, where the printing for the package is disposed between the laminated layers of plastic film, as well as in other application, it has been found to be desirable that the layers of plastic film be laminated together by the use of an adhesive. Although other methods of lamination are known, and some are used commercially, this invention is primarily directed to improving the existing art of making heat seals in composite plastic film which has been adhesively laminated.
It is considered to be conventional practice to use, as the laminating adhesive, an adhesive ink. That is, it has been commercial practice in some cases to use as the ink for printing, a material which is also an adhesive for the plastic films which are being laminated to form the composite film from which the ultimate bag or pouch will be formed. The adhesives, whether combined with ink or not, may suitably be pressure sensitive adhesives, or heat sensitive adhesives or for that matter substantially any other plastic laminating adhesive. The particular adhesives which are used to produce the laminates of this invention are considered to be per se conventional.
In practice, in order to save money, the plastic films have been getting progressively thinner. This has placed a strain on the weight holding capacity of the bags made from these films. Sometimes, the failure is at the heat seals which convert the composite films into the bags. This has led to a careful consideration of the seals, which has led to this invention.